But even several cells together do not produce the currents needed in
modern industry, and the flow is produced in a different manner. As the
invisible electrons pass along a wire they produce what we call a
magnetic field around the wire, they produce a disturbance in the
surrounding ether. To be exact, it is through the ether surrounding the
wire that the energy originated by the electrons is transmitted. To set
electrons moving on a large scale we use a "dynamo." By means of the
dynamo it is possible to transform mechanical energy into electrical
energy. The modern dynamo, as Professor Soddy puts it, may be looked
upon as an electron pump. We cannot go into the subject deeply here, we
would only say that a large coil of copper wire is caused to turn round
rapidly between the poles of a powerful magnet. That is the essential
construction of the "dynamo," which is used for generating strong
currents. We shall see in a moment how magnetism differs from
electricity, and will say here only that round the poles of a large
magnet there is a field of intense disturbance which will start a flow
of electrons in any copper that is introduced into it. On account of the
speed given to the coil of wire its atoms enter suddenly this magnetic
field, and they give off crowds of electrons in a flash.
It is found that a similar disturbance is caused, though the flow is in
the _opposite_ direction, when the coil of wire leaves the magnetic
field. And as the coil is revolving very rapidly we get a powerful
current of electricity that runs in alternate directions--an
"alternating" current. Electricians have apparatus for converting it
into a continuous current where this is necessary.
A current, therefore, means a steady flow of the electrons from atom to
atom. Sometimes, however, a number of electrons rush violently and
explosively from one body to another, as in the electric spark or the
occasional flash from an electric tram or train. The grandest and most
spectacular display of this phenomenon is the thunderstorm. As we saw
earlier, a portentous furnace like the sun is constantly pouring floods
of electrons from its atoms into space. The earth intercepts great
numbers of these electrons. In the upper regions of the air the stream
of solar electrons has the effect of separating positively-electrified
atoms from negatively-electrified ones, and the water-vapour, which is
constantly rising from the surface of the sea, gathers more freely round
the positively-electrified atoms, and brings them down, as rain, to the
earth. Thus the upper air loses a proportion of positive electricity, or
becomes "negatively electrified." In the thunderstorm we get both kinds
of clouds--some with large excesses of electrons, and some deficient in
electrons--and the tension grows until at last it is relieved by a
sudden and violent discharge of electrons from one cloud to another or
to the earth--an electric spark on a prodigious scale.
§ 11